This invention relates to epoxy resin compositions. In one aspect, the invention relates to the preparation of low-stress epoxy resin compositions useful in encapsulating electrical components.
Epoxy resins are used to encapsulate electrical components such as integrated circuits. The optimum properties for encapsulation resins include low melting point in the uncured state (to permit encapsulation of the component at a relatively low temperature), high glass transition temperature in the cured state (to permit encapsulation of a component which can tolerate high operating temperatures), low modulus and low coefficient of thermal expansion of the cured resin (properties related to the ability of the cured resin to resist "stress" or cracking at high operating temperatures), and fast cure (to permit maximum speed and productivity in assembly).
One class of epoxy resin used in electrical encapsulation is epoxidized orthocresol novolac (EOCN) resins. EOCN resins have high glass transition temperatures and good moisture resistance, but they are subject to "stress" or cracking under high operating temperatures.
Another class of epoxy resins suitable for electrical encapsulation are diglycidyl ethers of 4,4'-dihydroxybiphenyls, which are typically cured with phenol novolac-type curing agents. These epoxy systems exhibit the highly desirable property combination of low melt viscosity and low modulus, the latter property being an indication of the ability of the cured composition to resist cracking during high temperature use. However, for some encapsulation applications, the glass transition temperature of these resins is low and the cure rate is slow.
It is therefore an object of the invention to provide an electrical encapsulation formulation which cures rapidly, has low modulus and cures to a high glass transition temperature.